The present invention relates to the gettering of metals from a solution, and in particular, to the gettering of metals from an alkaline solution such as a semiconductor wafer cleaning solution.
The preparation of single crystal silicon wafers typically involves the steps of growing a single crystal ingot, slicing the ingot into wafers, and lapping, etching and polishing the wafers. Prior to being packaged for delivery to a customer, the polished wafers are routinely cleaned by a procedure commonly known as the RCA method developed by Kern. See W. Kern and D. Puotinen, RCA Rev. 31 (1970) 187. In this method the wafers are immersed in a series of alkaline and acidic baths. One of the alkaline baths ("SC-1") typically contains H.sub.2 O-H.sub.2 O.sub.2 -NH.sub.4 OH and one of the acidic baths ("SC-2") typically contains H.sub.2 O-H.sub.2 O.sub.2 -HCl. SC-1 which may range from about 1000:1:1 to 1:1:1 parts by volume H.sub.2 O:H.sub.2 O.sub.2 :NH.sub.4 OH, removes organic contaminants and particles by both the solvating action of ammonium hydroxide and the powerful oxidizing action of hydrogen peroxide. The ammonium hydroxide also serves to complex metals such as copper, gold, nickel, cobalt, zinc and calcium. SC-2 which may range from about 1000:1:1 to 1:1:1 parts by volume H.sub.2 O:H.sub.2 O.sub.2 :HCl, removes alkali and transition metals, and prevents redeposition from the solution by forming soluble metal complexes. F. Shimura, Semiconductor Silicon Crystal Technology (Academic Press; London, England; 1989), page 189.
Although the RCA method is relatively effective for cleaning particles from the surfaces of wafers, it is not without its disadvantages; SC-1 has been identified as a source of metal contamination. Silicon wafers bathed in a SC-1 cleaning bath prepared from ultrapure solutions typically have a surface contamination of about 1.times.10.sup.10 to about 1000.times.10.sup.10 metal atoms per square centimeter when the cleaning bath is fresh and the surface contamination will progressively increase as the number of wafers bathed in the cleaning bath increases. For this reason, SC-2 cleaning traditionally follows SC-1 cleaning but has proven to be only marginally efficient for removing Al and Fe. Significantly, however, current leading edge applications of silicon wafers require that the total of all surface metals be less than 1.times.10.sup.10 atoms per square centimeter. The present invention enables such metal contamination levels to be achieved.